M. R. Jennings

Field-effect mobility temperature modeling of 4H-SiC metal-oxide-semiconductor transistors

Here a physically based channel mobility model has been developed to investigate the temperature dependence of the field-effect mobility of 4H-SiC metal-oxide-semiconductor (MOS) transistors with thermally oxidized gate insulators. This model has been designed so that it accounts for the high density of traps at the MOS interface. This temperature dependence is a key issue for silicon carbide electronics, as its basic material properties make it the foremost semiconductor for high power/high temperature electronic devices in applications such as spacecraft, aircraft, automobile, and energy distribution. Our modeling suggests that the high density of charged acceptor interface traps, encountered in thermally grown gate oxides, modulates the channel mobility due to the Coulomb scattering of free carriers in the inversion layer. When the temperature increases, the field-effect mobility of these devices also increases, due to an increase in inversion charge and a reduction of the trapped charge. Experimental ...

Characterization and modeling of n-n Si/SiC heterojunction diodes

In this paper we investigate the physical and electrical properties of silicon layers grown by molecular beam epitaxy on 4H-SiC substrates, evaluating the effect of the Si doping, Si temperature deposition, and SiC surface cleaning procedure. Si∕SiC monolithic integration of Si circuits with SiC power devices can be considered as an attractive proposition and has the potential to be applied to a broad range of applications. X-ray diffraction and scanning electron microscopy are used to determine the Si crystal structure (cubic silicon) and morphology. I-V and C-V measurements are performed to evaluate the rectifying diode characteristics along with the Si∕SiC built-in potential and energy band offsets. In the last section, we propose that our Si∕SiC heteojunction diode current characteristics can be explained by an isojunction drift-diffusion and thermoionic emission model where the effect of doping concentration of the silicon layer and its conduction band offset with SiC is analyzed.

Modelling the inhomogeneous SiC Schottky interface

For the first time, the I-V-T dataset of a Schottky diode has been accurately modelled, parameterised, and fully fit, incorporating the effects of interface inhomogeneity, patch pinch-off and resistance, and ideality factors that are both heavily temperature and voltage dependent. A Ni/SiC Schottky diode is characterised at 2 K intervals from 20 to 320 K, which, at room temperature, displays low ideality factors (n   8), voltage dependent ideality factors and evidence of the so-called “thermionic field emission effect” within a T0-plot, suggest significant inhomogeneity. Two models are used, each derived from Tungs original interactive parallel conduction treatment of barrier height inhomogeneity that can reproduce these commonly seen effects in single temperature I-V traces. The first model incorporates patch pinch-off effects and produces accurate and reliable fits above around 150 K, and at current densities lower than 10−5 A cm−2. Outside this region, we show that resistive effects within a given patch are responsible for the excessive ideality factors, and a second simplified model incorporating these resistive effects as well as pinch-off accurately reproduces the entire temperature range. Analysis of these fitting parameters reduces confidence in those fits above 230 K, and questions are raised about the physical interpretation of the fitting parameters. Despite this, both methods used are shown to be useful tools for accurately reproducing I-V-T data over a large temperature range.

Micro and nano analysis of 0.2 Ω mm Ti/Al/Ni/Au ohmic contact to AlGaN/GaN

As GaN technology continues to gain popularity, it is necessary to control the ohmic contact properties and to improve device consistency across the whole wafer. In this paper, we use a range of submicron characterization tools to understand the conduction mechanisms through the AlGaN/GaN ohmic contact. Our results suggest that there is a direct path for electron flow between the two dimensional electron gas and the contact pad. The estimated area of these highly conductive pillars is around 5% of the total contact area.

High doped MBE Si p-n and n-n heterojunction diodes on 4H-SiC

The physical and electrical properties of heavily doped silicon (5x10^1^9cm^-^3) deposited by molecular beam epitaxy (MBE) on 4H-SiC are investigated in this paper. Silicon layers on silicon carbide have a broad number of potential applications including device fabrication or passivation when oxidised. In particular, Si/SiC contacts present several atractive material advantages for the semiconductor industry and especially for SiC processing procedures for avoiding stages such as high temperature contact annealing or SiC etching. Si films of 100nm thickness have been grown using a MBE system after different cleaning procedures on n-type (0001) Si face 8^o off 4H-SiC substrates. Isotype (n-n) and an-isotype (p-n) devices were fabricated at both 500 and 900^oC using antimonium (Sb) or boron (B), respectively. X-ray diffraction analysis (XRD) and scanning electronic mircorscope (SEM) have been used to investigate the crystal composition and morphology of the deposited layers. The electrical mesurements were performed to determine the rectifiying contact characteristics and band offsets.

Analysis of inhomogeneous Ge/SiC heterojunction diodes

In this article Schottky barrier diodes comprising of a n-n germanium-silicon carbide (Ge-SiC) heterojunction are electrically characterized Circular transmission line measurements prove that the nickel front and back contacts are Ohmic, isolating the Ge/SiC heterojunction as the only contributor to the Schottky behavior Current-voltage plots taken at varying temperature (IVT) reveal that the ideality factor (n) and Schottky barrier height (SBH) (Phi) are temperature dependent and that incorrect values of file Richardson constant (A**) are being produced, suggesting tin inhomogeneous barrier Techniques originally designed for metal-semiconductor SBH extraction are applied to the heterojunction results to extract values of Phi and A** that are independent of temperature. The experimental IVT data are replicated using the Tung model It is proposed that small areas, or patches, making Lip Only 3% of the total contact area will dominate the I-V results due to their low SBH of 1.033 eV The experimental IVT data are also analyzed statistically using the extracted values of Phi to build Lip a Gaussian distribution of barrier heights. Including the standard deviation and a mean SBH of 1.126 eV, Which Should be analogous to file SBH extracted from capacitance-voltage (C-V) measurements. Both techniques yield accurate values of A** for SiC. However, the C-V analysis did not correlate with the mean SBH as expected

Si/SiC bonded wafer: A route to carbon free SiO2 on SiC

This paper describes the thermal oxidation of Si/SiC heterojunction structures, produced using a layer-transfer process, as an alternative solution to fabricating SiC metal-oxide-semiconductor (MOS) devices with lower interface state densities (Dit). Physical characterization demonstrate that the transferred Si layer is relatively smooth, uniform, and essentially monocrystalline. The Si on SiC has been totally or partially thermally oxidized at 900–1150 °C. Dit for both partially and completely oxidized silicon layers on SiC were significantly lower than Dit values for MOS capacitors fabricated via conventional thermal oxidation of SiC. The quality of the SiO2, formed by oxidation of a wafer-bonded silicon layer reported here has the potential to realize a number of innovative heterojunction concepts and devices, including the fabrication of high quality and reliable SiO2 gate oxides.

Si/SiC heterojunctions fabricated by direct wafer bonding

The physical and electrical properties of Si/SiC heterojunctions formed by direct wafer bonding are presented. Atomic force microscopy (AFM) and imaging reveal an improved bonding quality when Si wafers are transferred to on-axis substrates as opposed to off-axis epitaxial layers. AFM analysis of the bonded wafer achieves a smoother surface when compared to molecular beam epitaxy-grown Si layers. A reduced roughness of only 5.8 nm was measured for bonded wafers. Current-voltage measurements were used to extract the rectifying characteristics of Si/SiC heterojunctions. These Si layers could lead to improved high quality and reliable SiO2 gate oxides

Enhanced field effect mobility on 4H-SiC by oxidation at 1500◦C

A novel 1500°C gate oxidation process has been demonstrated on Si face of 4H-SiC. Lateral channel metal-oxide-semiconductor-field-effect-transistors (MOSFETs) fabricated using this process have a maximum field effect mobility of approximately 40 cm\2 V-1 s-1 without post oxidation passivation. This is substantially higher than other reports of MOSFETs with thermally grown oxides (typically grown at the standard silicon temperature range of 1100-1200°C). This result shows the potential of a high temperature oxidation step for reducing the channel resistance (thus the overall conduction loss), in power 4H-SiC MOSFETs.

Nanoscale investigation of AlGaN/GaN-on-Si high electron mobility transistors

AlGaN/GaN HEMTs are devices which are strongly influenced by surface properties such as donor states, roughness or any kind of inhomogeneity. The electron gas is only a few nanometers away from the surface and the transistor forward and reverse currents are considerably affected by any variation of surface property within the atomic scale. Consequently, we have used the technique known as conductive AFM (CAFM) to perform electrical characterization at the nanoscale. The AlGaN/GaN HEMT ohmic (drain and source) and Schottky (gate) contacts were investigated by the CAFM technique. The estimated area of these highly conductive pillars (each of them of approximately 20-50 nm radius) represents around 5% of the total contact area. Analogously, the reverse leakage of the gate Schottky contact at the nanoscale seems to correlate somehow with the topography of the narrow AlGaN barrier regions producing larger currents.